Enhancement: Allow specifying percentage in `check --read-data-subset`

We've enhanced the `check` command's `--read-data-subset` option to also accept
a percentage (e.g. `2.5%` or `10%`). This will check the given percentage of
pack files (which are randomly selected on each run).

https://github.com/restic/restic/issues/2186
https://github.com/restic/restic/pull/3038
